A centrifuge rotor is a relatively massive member used within a centrifuge instrument to expose a liquid sample to a centrifugal force field. The rotor is provided with a plurality of cavities in which containers carrying the liquid sample are received. The rotor has a central, axial mounting recess provided therein, whereby the rotor may be mounted to a shaft extending from a source of motive energy.
The possibility exists that, in use, the rotor may break apart due either to (i) fatigue failure of the rotor material, (ii) the imposition of excessive centrifugally induced stresses when the rotor is rotated past its predetermined rated speed (overspeed failure), or (iii) failure from the accumulated effects of corrosion caused by sample spillage. A failure produces a number of rotor body fragments each of which carries a portion of the kinetic energy of the rotor. A containment system is provided in the centrifuge instrument in order to contain the resultant rotor body fragments within the confines of the instrument, thus avoiding damage to people and/or property.
The size of the fragments usually depends upon the cause of the rotor failure. In a rotor failure caused by corrosion, for example, the fragments are relatively small, because the region of the rotor affected by corrosion is the sample receiving cavity near the rotor periphery. Rotor failure caused by fatigue or overspeed may be more severe.
The most severe form of rotor failure is a so-called "bi-hub" failure, in which the rotor breaks into two relatively massive fragments. The origin of the failure in a bi-hub failure is usually in the vicinity of the rotor mounting recess. In such a failure, the containment system is designed to contain the fragments within the instrument such that the impact of the fragments may cause movement of the instrument in the laboratory.
Various forms of mechanical arrangements are known which minimize the possibility of rotor failure due to overspeed. One class of overspeed protection arrangement includes a frangible member which fractures when an overspeed condition is imminent to mechanically disconnect the rotor from its source of motive energy. U.S. Pat. No. 3,990,633 (Stahl), U.S. Pat. No. 4,568,325 (Cheng et al.), U.S. Pat. No. 4,753,630 (Romanauskas), and U.S. Pat. No. 4,753,631 (Romanauskas), the latter two patents being commonly assigned herewith are representative of this class of overspeed protection arrangement. Another overspeed protection arrangement generally of this form includes a frangible member which fractures when an overspeed condition is imminent to electrically disconnect the rotor from its source of motive energy. U.S. Pat. No. 3,101,322 (Stallman) is representative of this form of arrangement.
Another known overspeed protection arrangement also uses a frangible element on the rotor which fractures when rotor speed reaches a predetermined value. The fragment so produced causes the rotor to be braked by increasing windage within the chamber in which the rotor is carried or by mechanical friction with the surrounding structure, thereby slowing rotor speed. Representative of this class of overspeed protection arrangement are U.S. Pat. No. 4,693,702 (Carson et al.), U.S. Pat. No. 4,132,130 (Schneider), U.S. Pat. No. 4,509,896 (Linsker), and U.S. Pat. No. 4,507,047 (Coons).
Other arrangements are known which minimize the possibility of rotor failure due to fatigue of the material. One form of such a rotor protection arrangement limits the stress produced in the vicinity of the mounting of the rotor to the shaft. U.S. Pat. No. 4,822,330 (Penhasi) is believed exemplary of this class of device. DE-A-3806284 (Hirsch) discloses a centrifuge rotor having portions of the undersurface removed to reduce stress in the rotor.
Another alternative to control the effects of rotor failure is to design a rotating apparatus, as a flywheel, to exhibit predetermined areas of vulnerability of rupture. The area of vulnerability may be defined by regions of weaker material or stress risers in the material of the flywheel. Thus, in the event of an overspeed, failure will most likely occur in the area of vulnerability, producing a fragment having a predictable mass. U.S. Pat. No. 3,662,619 (Seeliger) and U.S. Pat. No. 4,111,067 (Hodson) are believed exemplary of this class of device.
Still another alternative to controlling rotor failure is set forth in U.S. Pat. No. 5,279,538 (Carson), wherein a centrifuge rotor is characterized by a portion of the undersurface thereof being removed to define a predetermined number of bosses, each with a sample receiving cavity therein.
U.S. Pat. No. 5,562,554 (Carson) discloses a centrifuge rotor having a central hub with a mounting recess therein, a ring disposed concentrically about the hub, the ring having a plurality of cavities formed therein, and a relatively thin web connecting the hub and the ring. The web defines a localized region which exhibits a stress therein that is greater than the stress present in any other portion of the rotor when the rotor is operating at the predetermined operating speed, so that, over operation time, the probability that rotor failure will occur only in the web is enhanced.
Occasionally, rotors from a centrifuge break apart due to the high speeds and centrifugal forces to which they are exposed. In a bi-hub failure of a fixed angle centrifuge rotor, such as that set forth in FIG. 1, the rotor body fragments will have a transitional and rotational velocity.
In centrifuge 100 shown in FIG. 1, after contacting the centrifuge chamber 140 and guard ring 130, the rotor body fragments will travel in a vertical direction and come into contact with lid 180 which could prove to be potentially harmful should lid 180 not be able to contain such a force imparted by a rapidly moving rotor body fragment. A typical centrifuge 100, as shown in FIG. 1, includes an outer casing 120 which completely surrounds a centrifuge chamber or bowl 140. Disposed between centrifuge chamber 140 and outer casing 120 is guard ring 130. Casing 120 is provided with an access opening 160 through which rotating elements are inserted for centrifugation of their contents within centrifuge 100. Access opening 160 is covered by a suitable door or lid 180 when centrifugation is in progress. Centrifuge chamber 140 is mounted within casing 120 by any suitable support arrangement such as that shown in FIG. 1 in which casing 120 has inwardly depending shoulders 200 which receive a lip or flange 220 disposed circumferentially about the top portion of centrifuge chamber 140. Of course, any suitable mounting arrangement may be utilized.
Container 140 is typically defined by a substantially cylindrical sidewall portion 240 having an annular floor or bottom 260 connected thereto. In practice, sidewall 240 and floor 260 are formed integrally one with the other by a stamping operation. Located centrally and axially of floor 260 of container 140 is an opening 280 defined by a curled back lip portion 300. Lip 300 extends upwardly into the volume defined on centrifuge chamber 140. The exterior surface of sidewall 240 may be provided with impact absorbing shielding or guard ring 130 and/or evaporator coils (not shown) if centrifuge 100 is a refrigerated centrifuge.
Extending upwardly along the central axis of container 100 and projecting into the region or volume defined on the interior of container 140 is a gyro shaft 320. Shaft 320 is supported by suitable bearings with a rotor gyro 340. Gyro 340 is supported from an abutment 360 mounted to casing 120. A rubberized boot 380 received by lip 300 closes the space between lip 300 and gyro 340.
Rotational force is imparted to shaft 320 (and to a rotating element mounted thereon) connected by a pulley and belt 400 with a source of motive energy shown as a motor 420. The direction of rotor rotation is shown by the arrow `W` about the spin axis 500. The upper end portion of shaft 320 is provided with a spud 440 adapted to receive thereon the central hub of a rotating element, or rotor 460F, having a correspondingly configured central axial well therein. The rotor, when placed and secured to spud 440, is thereby mounted for rotational movement within centrifuge chamber 140.
When the rotor breaks apart into fragments, the fragments have a translational velocity substantially in a radial plane and a rotational velocity substantially parallel to the spin axis. When a fragment strikes the wall of the chamber or guard ring, the fragment will continue to rotate about the strike point in a direction substantially parallel to the spin axis. This rotation is caused by the moment at impact due to the center of gravity of the fragment not being on the same plane as the point of the rotor that first strikes the wall of the centrifuge chamber or guard ring.
The present inventors have discovered that if the center of gravity of a rotor body fragment is above a predetermined strike point plane, the fragment moves toward the centrifuge door or lid. This is due to the rotational velocity of the fragment and the moment created about the center of gravity of the fragment which cause it to move upwards toward the centrifuge door, typically in a spiral-type pattern. The present inventors have also discovered that one method of minimizing the challenge to the door would be to control the position of the rotor body fragment's center of gravity in relation to the strike point which is typically on the rotor's outer most edge or largest diameter. If the rotor body fragment's center of gravity is below the strike plane, then the fragment moves toward the floor of the centrifuge chamber. Downward movement of the fragment causes the centrifuge device to transmit a downward force against the floor or table top which absorbs such a shock rather than against the centrifuge door or lid which is less desirable.
A second method to minimizing the challenge to the door would be to configure the shape or strength of the centrifuge chamber or guard ring to counteract the effect of the moment at impact thereby preventing the fragment from moving toward the centrifuge lid.
The present invention also provides many additional advantages which shall become apparent as described below.